Slow post-synaptic transmitter actions are important as a means of achieving long lasting changes in the electrical activity of nerve and muscle cells and are of interest as they often involve novel ionic and molecular mechanisms. Here I propose to study, in central neurons of the marine snail Aplysia, two slow transmitter actions that have been shown to mediate presynaptic facilitation and presynaptic inhibition, two forms of plastic changes in synaptic effectiveness: 1. Serotonin (5-HT) and stimulation of a putative serotonergic interneuron elicit a slow excitatory post-synaptic potential and facilitate transmitter release from a cluster of mechanoreceptor sensory neurons by a decrease in a specific membrane K+ conductance via cAMP-dependent phosphorylation. This increase in synaptic efficacy is thought to underlie behavioral sensitization, a simple form of learning. 2. Histamine and the stimulation of a putative histaminergic neuron produce a slow inhibitory post-synaptic potential and inhibit transmitter release from the cholinergic neuron L10 due to a decrease in the inward membrane calcium current. In this study, the recently developed patch clamp technique will be applied to investigate these transmitter actions at the level of single ion channel function. The research should provide insight into a number of general and specific questions, including: 1. What are the characteristics of transmitter action on single channel currents. 2. What are the biophysical properties of the individual potassium and calcium ion channels that underlie the action of serotonin and histamine, respectively. 3. What is the role of cAMP-dependent phosphorylation in channel modulation and what is the molecular mechanism of this modulation. An understanding of the basic mechanisms of a particular cAMP mediated transmitter action is likely to provide insight into such transmitter actions in general, including those occuring in the central nervous system of higher vertebrates.